cold weather growing

quite a few places in the USA did not get a proper summer this year,
and now it is already winter there,
it appears to me to be the new weather from at least a mini ice age
the food supply is the big issue when that happens, 
and you can grow food if you know what to grow,
so let's assume that is happening for the moment,
so how do we get ready ?

the new weather requires something that is frost tolerant, and can deal with low CO2 levels,
many current crops are already having issues from low CO2, just wait till the levels start dropping and see what happens,
there are 3 kinds of plants as far as dealing with CO2
they are called C3, C4, and CAM
C3 are most plants out there, and 160 PPM CO2 is the lower limit where they will just stop growing, this is all the wheat, oats, trees, ect...
C4 plants will collect the CO2 and can deal with quite low levels, corn and sorghum are the big ones here, but they can't take frost,
CAM plants are some cactus, they gather CO2 at night and photosynthesize in the day, pineapple and agave are pretty much the only foods this applies to

went and looked for any plants that are a C4 type that can take frost, turns out there is one groop, it is the brassica family
so that is cabbage, cauliflower, broccoli, Brussels sprouts, rutabaga, turnips, kohlrabi, bok choy, ect...
now lets go look at the diet of people in the mini ice age,
and it turns out to be lots of brassica plants...
no mystery why anymore

I suggest getting seeds for them.
potatoes and wheat will also end up doing pretty well for the first few hundred years before the CO2 drops to far